1. Field of the Invention
The present invention generally relates to drilling systems and methods.
2. Background Technology
A cased drilling system may be used for drilling into a formation to form a borehole in the formation. The cased drilling system may include an inner drill string and an outer drill string. The inner drill string may include an inner drill bit and/or one or more drill rods, and the outer drill string may include an outer drill bit and/or one or more casings. For example, the inner drill string may include an inner drill bit that may be connected to a series of connected drill rods, and the outer drill string may include an outer drill bit that may be connected to a series of connected casings. The series of connected drill rods and the series of connected casings may be assembled section-by-section as the inner and outer drill strings move deeper into the formation.
The inner drill string and the outer drill string may be used independently to drill into the formation. In further detail, the inner drill string and/or the outer drill string may be used to drill into the formation until, for instance, the outer drill string reaches a desired depth in the formation. After the outer drill string has reached a desired depth into the formation, the outer drill string may remain in place, while the inner drill string drills farther into the formation. For example, the inner and/or outer drill strings may be used to drill into the formation through less stable ground that could risk a borehole collapse (e.g., disturbed ground, sand, soft clay, boulders or other less stable ground conditions that could be washed out when a flushing media is pumped into the borehole) and into more stable ground. After the outer drill string has reached a desired depth into the more stable ground (e.g., about one or two meters or other desired depth), the outer drill string may remain in place, while the inner drill string drills farther into the formation and/or past a leading end of the outer drill string. By extending through the less stable ground and into the more stable ground, the outer drill string may advantageously help reduce the risk of a collapse of the borehole.
When the outer drill string reaches the desired depth in the formation, the inner drill string and/or the outer drill string may be disconnected from a head, a top hammer, and/or other driving mechanism used to advance the drill strings into the formation. With the drill strings disconnected from the driving mechanism, a conduit (sometimes referred to as a “bottom preventer”) may be connected to a trailing or upper portion of the outer drill string. In particular, a trailing or upper portion of the inner drill string may be inserted through a leading or lower portion of the conduit, through the conduit, and out a trailing or upper portion of the conduit. Then, the conduit may be connected to the trailing or upper portion of the outer drill string, and the inner drill string may be reconnected to the driving mechanism for use in drilling farther into the formation. The conduit may be sized and configured to be engaged by a clamp while the conduit is connected to the outer drill string, which may help prevent inadvertent loss of the outer drill string down the borehole.
As the inner drill string drills farther into the formation, portions of the inner drill string may enter into the trailing or upper portion of the conduit, pass through the conduit, exit the leading or lower portion of the conduit, and into the outer drill string. The conduit may include one or more seals configured to form a seal with the portions of the inner drill string passing through the conduit.
While the inner drill string and/or the outer drill string drill into the formation, a flushing medium (such as water, air or other flushing medium) may be flushed through the inner and outer drill strings, which may remove debris (e.g., portions of the formation and/or portions of the drill bits) from the borehole and the inner and outer drill strings. In further detail, the inner drill string and the outer drill string may be tubular. For example, the rods of the inner drill string and the casings of the outer drill string may be tubes. The flushing medium may be pumped into the trailing or upper portion of the inner drill string, down through the inner drill string, out a leading or lower portion of the inner drill string, into a lower portion of the borehole, into a gap formed between an inner surface of the outer drill string and the outer surface of the inner drill string, up through the gap between the outer and inner drill strings, and out one or more outlets. When both of the drill strings are connected to the driving mechanism, these one or more outlets may include one or more outlets formed in another conduit, such as a “top preventer.” Alternatively, these one or more outlets may include one or more outlets formed in the “bottom preventer” when connected to the trailing or upper portion of the outer drill string. The bottom preventer's seal(s) may be formed above its outlet(s), which may help guide the flushing medium and the debris conveyed by the flushing medium out of the bottom preventer via the outlet(s). In some instances, the flushing medium and the debris may be under high pressure and/or may move at a high speed through the drill strings. In such instances, the bottom preventer may advantageously prevent the flushing medium and the debris from spewing violently and uncontrollably out of the borehole and, instead, may guide the flushing medium and the debris through the bottom preventer's outlet(s) and away from the drilling system. For example, one or more hoses may be connected to the bottom preventer's outlet(s) to guide the flushing medium and the debris away from the drilling system. Consequently, the bottom preventer may help avoid creating accidents, environmental hazards, and messes at job sites.